Project information

Project financing:

Deutsche Forschungsgemeinschaft

Abstract

In this thesis, crystal structures of the two organic molecular compounds trimethyltin hydroxide (Me3SnOH) and
Lambda-Co(III) sepulchrate trinitrate (Co(sep)(NO3)3)
are reported for different temperatures. Interplay between intermolecular and intramolecular forces and between different intermolecular forces are found to be the origin of modulation in these crystal structures. Employing the (3+1)-dimensional superspace formalism unique phase relations have been established between non-modulated phases and different modulated phases. Description of high Z' superstructures (where Z' is the number of molecules in the asymmetric unit) as commensurately modulated structures
within the superspace formalism helped in removing correlations between structural parameters and in establishing complete physically and chemically meaningful
structural models which are often problems in three-dimensional descriptions.
Me3SnOH undergoes a phase transition from high Z' phase to low Z' phase upon cooling. Crystal structures of the low Z' phase and high Z' phase have been redetermined. Single-crystal X-ray diffraction experiments have been performed at
T1 = 100 K and T2 = 220 K employing synchroton radiation.
One of the major discoveries is observation of diffuse scattering in layers in the diffraction pattern in both phases. Based on this observation the description of the high Z' phase reduced from 32-fold superstructure to eightfold superstructure in contrast to literature published earlier. Crystal structures of both phases have been described within (3+1)-dimensional superspace formalism
as commensurately modulated structures using basic cell with Z' = 1/2. Due to orthorhombic symmetry the description of the corresponding a x b x 2c superstructure reduces to Z' = 1 and of the a x b x 8c superstructure reduces to Z' = 4.
In the Z' = 1 superstructure inter-strand C-H...O bonds are discovered. These weak hydrogen bonds serve as pivot for the crystal packing and zigzag arrangement of the polymeric chains. The origin of multiple formula units (Z' = 4) in the eightfold superstructure in the high Z' phase is argued to lie in the competition between dense packing of the formula units within the chain and optimal conformation of the hydroxy groups. This frustration within the polymeric chain is responsible for the distortion of zigzag planes.
Intra-strand competitive forces and inter-strand H...H interactions prevent the formation of long range order in inter-strand C-H...O bonds. The disorder in both forms is proposed as shift of the chains with respect to another
and arbitrary direction of the zigzag chains.
Complete scheme of phase transitions of Co(sep)(NO3)3 is investigated. Single crystal X-ray diffraction experiments have been performed at T1 = 115 K, T2 = 100 K and T3 = 95 K.
Phases II (dT = 133-107 K) and III (dT = 107-98 K) are found incommensurately modulated and phase IV (T < 98 K) is commensurately modulated. The hexagonal symmetry of the high temperature phase is reduced in phase II, III and IV
and six monoclinic twin domains are found in the crystal.
The incommensurately modulated structure in phase III and
commensurately modulated structure in phase IV are described within (3+1)-dimensional superspace approach using (monoclinic c-unique) superspace group C21(σ1σ20)0. The corresponding three-dimensional structure in phase IV is found to be a twelvefold superstructure (Z' = 12) of the room temperature hexagonal structure. Employing the superspace approach large correlations between structural parameters could be removed and save approximately 1/5 of refinable parameters in comparison to the three-dimensional superstructure model. Crystal structures in phase III and in phase IV are completely ordered owing to dense packing of the cations and anions in contrast to the room temperature structure. The origin of modulation is argued to lie in the avoidance of repulsive C-H...O interactions
rather than formation of attractive N-H...O bonds
between Co(sep) cages and two nitrate groups. The effect of modulation is largest at the site of the third nitrate group driven by C-H...H--C interactions between Co(sep) cages in the co-ordination sphere and C-H...O bonds between
this nitrate group and Co(sep) cages.